The present invention relates to improved gaskets used for shielding electronic devices from electromagnetic interference or radio frequency interference (EMI or RFI), and for environmental sealing. Such improvements include enhanced resistance to corrosion and a reduction in the compressive forces needed to create and maintain the seal.
The operation of electronic equipment, such as televisions, radios, computers, medical instruments, business machines, communication equipment, and the like, is typically accompanied by the generation of radio frequency and/or electromagnetic radiation within the electronic circuits of the electronic system. The increase in operating frequencies in commercial devices utilizing electronic enclosures, such as doors and access panels, housings for shielding computer cabinets and drives, cathode ray tubes (CRTs) and automotive electronic modules, results in an elevated level of high frequency electromagnetic interference (EMI). Any gap between the metal surfaces confronting or mating with the doors and access panels affords an opportunity for the passage of electromagnetic radiation and the creation of electromagnetic interference. These gaps also interfere with the electric currents running along the surfaces of the cabinets from EMI energy, which is absorbed and conducted to the ground.
If not properly shielded, RFI and EMI radiation can cause considerable interference with unrelated electronic equipment. Accordingly, it is necessary to effectively shield and ground all sources of radio frequency and electromagnetic radiation within the electronic system. Therefore, it is advisable to use a conducting shield or gasket between such surfaces to block the electromagnetic interference.
To attenuate EMI effects, shielding gaskets having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other “target” devices from other source devices. Such shielding is provided as a barrier inserted between the source and the other devices, and is typically configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, gaps may be present which reduce the efficiency of the shielding by containing openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals can be bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path there across by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally should be wear resistant, economical to manufacture, and capable of withstanding repeated compression and relaxation cycles.
Materials employed to fabricated EMI gaskets commonly include, for instance, various polymers containing electrically conductive metal particles, such as particles of copper, nickel, silver, aluminum, tin, or various conductive alloys of these metals. Other conductive particles and fibers such as carbon, graphite, or conductive polymeric materials may be substituted for the metal particles. Alternatively, EMI gaskets can be formed from wires encapsulated in resilient polymeric materials, such as elastomers or foam rubber.
The above-described gaskets exhibit a number of problems in actual use, such as corrosion of the metallic wire portion of the gasket, and the failure of the elastomer to provide the desired environmental seal. The use of noble metal wires adds significantly to the cost of the gasket and does not always solve the corrosion and oxidation problems. Additionally, a moisture leak path may be formed at the point where the wires contact the sealing metal surfaces if the elastomer does not adequately seal around and between the wires or metallic surfaces to prevent the migration of moisture into or through the gasket area. This may result in corrosion or other problems in the electrical or electronic device being protected by the gasket. These problems are exacerbated in high performance applications, such as aircraft applications, where the seal is required to perform in difficult environments.
Accordingly, it is frequently the case that gaskets providing satisfactory EMI shielding capabilities are unable to provide environmental sealing, thereby causing corrosion problems. Alternatively, gaskets which provide adequate environmental sealing frequently fail to provide the desired EMI shielding capabilities. As a result, the EMI shielding and environmental sealing functions have typically been seen as two separate functions requiring two separate products. For instance, a gasket designed for environmental sealing can include an external conventional environmental seal, such as an elastomeric or rubber O-ring, coupled with an interior EMI gasket, such as a wire mesh. This arrangement is undesirable because it is bulky, requires extra design, engineering and machining, increasing installation difficulties and adding to the cost and risks of proper installation.
Representative gaskets and seals developed by others are described below by way of illustration and comparison.
U.S. Pat. No. 2,477,267 discloses EMI shielding gaskets for placement between adjacent metallic surfaces. The gaskets are wire meshes or screens impregnated with a suitable elastomer as an environmental shield to create a non-porous structure. Electrical contact between the adjacent metallic surfaces is provided by contact with the high points of the mesh which are exposed through the elastomer coating.
U.S. Pat. No. 4,900,877 is also directed to EMI gaskets formed from wires or metal filaments which utilize a gel material to seal the space between adjacent metallic surfaces. The wire mesh is encapsulated in the polymeric gel to provide an environmental seal and to reduce corrosion.
U.S. Pat. No. 3,140,342 describes metal-filled conductive plastic sheets which are used as EMI shielding gaskets. The gaskets of the reference are composite structures having a compressible non-conductive plastic core which reinforces the conductive layer to provided additional resiliency for the gasket.
U.S. Pat. No. 6,173,970 is directed to composite gaskets comprising a non-conductive silicone sponge for environmental protection, and a metal-filled conductive silicone gasket for EMI shielding. The conductive composite gasket of the reference is adapted for placement between adjacent electronic parts having a tongue and groove configuration.
U.S. Pat. No. 6,231,055 describes another composite strip gasket design comprising a block of gel sealant and an interlocking carrier member. The gasket is adapted to be inserted and locked in place in a cavity formed in an electronic device to provide shielding for the device.
U.S. Pat. Nos. 6,454,276 and 6,719,293 are directed to composite, multi-layered gaskets for use in aircraft to provide corrosion resistance and EMI shielding for external aircraft electronics. The gasket design includes sheets of conductive wire mesh encapsulated with a fluorosilicone compound that provides enhanced corrosion resistance in such external applications.
U.S. Pat. No. 6,497,414 discloses a sealing element design with radially projecting ribs for supporting and protecting fiber optic cables. The sealing element can include sections formed from a gel material for environmental protection.
The respective disclosures of each of the patents listed above are incorporated by reference herein in their entireties.
It is accordingly an objective of the present invention to provide an improved environmental seal having a high level of EMI shielding as well as resistance to environmental corrosion and degradation for use in difficult sealing environments.